Extracellular Production of Hydrogen Selenide Accounts for Thiol-assisted Toxicity of Selenite against Saccharomyces cerevisiae

Agathe Tarze,Marc Dauplais,Ioana Grigoras,Myriam Lazard,Nguyet-Thanh Ha-Duong,Frédérique Barbier,Sylvain Blanquet,Pierre Plateau

doi:10.1074/jbc.m610078200

Abstract

Administration of selenium in humans has anticarcinogenic effects. However, the boundary between cancer-protecting and toxic levels of selenium is extremely narrow. The mechanisms of selenium toxicity need to be fully understood. In Saccharomyces cerevisiae, selenite in the millimolar range is well tolerated by cells. Here we show that the lethal dose of selenite is reduced to the micromolar range by the presence of thiols in the growth medium. Glutathione and selenite spontaneously react to produce several selenium-containing compounds (selenodiglutathione, glutathioselenol, hydrogen selenide, and elemental selenium) as well as reactive oxygen species. We studied which compounds in the reaction pathway between glutathione and sodium selenite are responsible for this toxicity. Involvement of selenodiglutathione, elemental selenium, or reactive oxygen species could be ruled out. In contrast, extracellular formation of hydrogen selenide can fully explain the exacerbation of selenite toxicity by thiols. Indeed, direct production of hydrogen selenide with D-cysteine desulfhydrase induces high mortality. Selenium uptake by S. cerevisiae is considerably enhanced in the presence of external thiols, most likely through internalization of hydrogen selenide. Finally, we discuss the possibility that selenium exerts its toxicity through consumption of intracellular reduced glutathione, thus leading to severe oxidative stress.

Highlights

Selenium deserved additional interest when an inverse correlation between cancer mortality rates and geographic distribution of selenium in forage crops was observed [2]
Because the cancer-protective effects keep on increasing at selenium intake levels exceeding that required to maximize the activity of selenoproteins [11], it is unlikely that selenoproteins alone mediate the protection against cancer
Glutathione Exacerbates the Toxicity of Selenite toward S. cerevisiae Cells—To evidence an effect of glutathione on the sensitivity of S. cerevisiae to selenite, cell survival was estimated after 1-h incubation in Synthetic dextrose (SD) minimal medium containing varying concentrations of both glutathione (0 – 400 ␮M) and sodium selenite (0 –50 ␮M)

Summary

Introduction

Selenium deserved additional interest when an inverse correlation between cancer mortality rates and geographic distribution of selenium in forage crops was observed [2]. The boundary between cancer-protecting and toxic levels of selenium is extremely narrow. The mechanisms of selenium toxicity remain largely unknown and need to be further investigated. To this end, we chose the yeast Saccharomyces cerevisiae to study the effect of selenite on viability. We chose the yeast Saccharomyces cerevisiae to study the effect of selenite on viability This model organism has the advantage to be devoid of selenoproteins [13]. To elucidate the underlying mechanisms, we have investigated the capacity of S. cerevisiae cells to withstand exposure to selenite upon addition to the growth medium of glutathione, L-cysteine, or dithiothreitol. Several selenium-containing compounds as well as reactive oxygen species derive from the spontaneous reduction of selenite by thiols. Thiol-assisted Toxicity of Selenite product of the reduction of selenite, must be at the origin of the thiol-assisted selenite toxicity

Results

Discussion

Conclusion